There are two behaviors that stop such a good practice from being followed.
1. Product managers thinking that design is a creative activity for which they are not suited. Really? If you are not creative, why are you managing a product?
2. User experience designers considering design as a unique trait that only they can do and having this myopic view that experience design is product design.
Product managers who don't make things to test the waters are doomed to fail sooner or later. User experience designers who do not recognize this and embrace it are are going to fail because they will end up designing products that may not sell well.